1. Field of the Invention
This invention relates to a vehicle speed control device for use in a trolley-assisted dump truck of a single phase alternating current system, and more particularly to the vehicle speed control device which can vary an output of an electric motor for driving the dump truck by varying a voltage applied to the motor in proportion to the degree of depression of an acceleration pedal.
2. Description of the Prior Art
In view of the availability of petroleum becoming tight increasingly in the recent years, there is a relevant tendency towards energy saving even in large mines. In particular, in the districts where sufficient power can be generated by hydraulic power and by thermal power obtained by burning coals, but are lacking in petroleum resources, there can be seen a strong demand for replacing petroleum driven mining machines by electrically driven mining machines in order to save petroleum products.
Besides the tendency of petroleum saving, there can be seen a tendency of falling productivity due to the suspension of movements of vehicles so as to prevent a pit of the mine from oxygen deficient condition arising from the exhaust gas smog emitted from the engines of ore carrying vehicles and the amount of which increases as the pit becomes deeper.
In order to eliminate such troubles in the late years, there has been proposed a double-wire trolley assisted dump truck system utilizing a cheap commercial power supply on the ascent courses extending from the bottom of the pits to the dumping areas.
The trolley-assisted dump trucks can be driven either in a trolley mode utilizing a commercial power source or in an engine mode utilizing generators driven by internal combustion engines mounted on themselves, respectively.
A commercial power source is utilized to run dump trucks in the trolley mode and the voltage applied across double trolley wires is maintained constant, and therefore the output of the drive motor can be kept constant. For this reason, it has so far been impossible for the operator to control the torque for driving dump trucks as desired.